By-passing or Removing Passwords

I have a toshiba laptop which was given to me by a pal. He has 2 users accounts, all passworded and I only log in as Guest. I need to access other programs and install new ones, but I’ve been denied the rights. How can I bypass or remove those passwords so I can access my laptop fully? The guy has forgotten his own passwords. Somebody help me pleaseeeeeeeeeeeeeeeeeee!